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How Indian forensic labs work paint: composition, automotive multilayer coatings, FTIR and Py-GC-MS, SEM-EDX pigment chemistry, and the PDQ database in hit-and-run cases.
Paint evidence is the second workhorse of vehicular forensics, sitting alongside glass in every hit-and-run kit Indian SFSLs work through. A flake the size of a grain of rice can carry four or five stratified layers (clearcoat, basecoat, primer surfacer, electrocoat primer, sometimes a factory anti-corrosion layer), and each layer is itself a tiny chemistry experiment: a polymer binder, dispersed pigments, extenders, additives. Read those layers correctly and you can place a suspect vehicle on the road, often to make and model, sometimes to a single repaint shop in a single city.
The contrarian point is that paint is doing more evidentiary work than most Indian IOs realise. Glass tells you a vehicle was there. Paint tells you which vehicle, because the layer sequence varies between OEMs, between model years from the same OEM, and between original-factory and refinish jobs. The PDQ (Paint Data Query) database run by the RCMP catalogues automotive paint systems from manufacturers worldwide and is the international reference for matching a paint flake to a make/model/year range. Indian SFSLs don't operate a domestic equivalent at scale yet, but they do query PDQ through bilateral arrangements on flagship cases. What survives cross-examination isn't the colour match. It's the layer-by-layer stratigraphic correspondence.
Four ingredients, but the ratios decide everything.
A paint film is a cured polymer matrix with dispersed solid and liquid components. Four ingredient classes do the work:
Three families, three lab workflows, three sets of expected layer counts.
The three families of paint that turn up in Indian SFSL casework break down by where the paint was applied and how many layers it carries.
| Property | Architectural (house, wall) | Automotive OEM | Automotive refinish | Industrial |
|---|---|---|---|---|
| Typical layer count | 1 to 3 | 4 to 5 | 2 to 4 | 1 to 3 |
| Standard binder | Acrylic, alkyd, emulsion | Polyurethane / acrylic-melamine | Polyurethane, 2K acrylic | Epoxy, polyurethane, alkyd |
| Primer layer | Often absent on touch-ups | E-coat + primer surfacer | Often a single primer | Zinc-rich or epoxy |
| Top layer | Single coat, matt or sheen | Clearcoat (unpigmented) | Clearcoat (when present) |
Substrate up to clearcoat, in the order the factory applied them.
A modern Indian-market passenger car (post-2010 Maruti, Hyundai, Tata, Mahindra factory production) carries a paint system applied in a fixed sequence on the body-in-white line. Each layer has a specific function and a specific chemistry, and reading the layers in cross-section under a stereomicroscope is the first thing the SFSL Trace division does with a recovered flake.
A staged workflow that gets to definitive ID without burning sample mass.
Paint analysis at an Indian SFSL is sequenced from least to most destructive. The first pass is microscopy, the second is FTIR, the third is Py-GC-MS or SEM-EDX depending on the question. The sample is irreplaceable, and burning it on a thermal technique before microscopy is the kind of error that ends careers.
What each instrument actually answers, and where each one runs out.
Each instrument has a question it answers cleanly and a question it cannot. Knowing the boundary is what separates a competent paint examiner from one who over-claims on the witness stand.
From kerb to lab, and where Indian infrastructure has gaps.
Paint collection follows a small set of rules that prevent the most common evidentiary failures. The principles are summarised in the broader scene-processing workflow; the paint-specific points sit below.
The Indian hit-and-run workflow ties paint and glass together. A typical case: a pedestrian fatality on a state highway, no eyewitness, a windscreen-glass cluster and a paint smear at the scene. SFSL findings: glass RI 1.518, density 2.501 g/cm³, ICP-MS trace profile narrows to a Toyota Innova production batch; paint flake recovered from the victim's jacket shows four-layer OEM stack with polyurethane clearcoat, acrylic-melamine basecoat in metallic silver, polyester primer surfacer, black epoxy e-coat; PDQ query returns Toyota Innova 2018 to 2021 in Toyota Bidadi paint specification 1G3. A vehicle matching the description is recovered three days later from a service station; physical fit on the glass plus stratigraphic correspondence on the paint plus matching basecoat pigment chemistry carries the conviction.
In a modern automotive paint system, the layer applied directly to the bare metal substrate is:
| Often single pigmented top |
| Frequency in Indian casework | Common in property crime, arson | Dominant in vehicular cases | Common in commercial-vehicle hit-and-runs | Less common; theft from industrial premises |
A scene-team SOCO who can name the paint family from a stereomicroscope view at 10 to 40x is doing the same work the lab will repeat at intake. Architectural paints are short stacks of similar layers. Automotive OEM is a four-or-five-layer sandwich with a clear, structured sequence. Refinish work is a shorter stack laid over an existing OEM stack, often with visible boundary contamination. Industrial paints are usually pigmented monocoats or two-layer systems over zinc-rich primer.
Substrate up, the layers are: bare metal, e-coat, primer surfacer, basecoat, clearcoat.
Refinish work shortcuts this sandwich. A typical body shop repaint over an existing OEM stack adds a primer (sometimes), a basecoat, and a clearcoat, for a total layer count of 7 to 9 in cross-section. The boundary between the original OEM clearcoat and the refinish primer is usually visible under 100x as a sharp interface with trapped contamination (dust, sanding residue). Indian appellate courts have accepted refinish-vs-OEM determinations made on these stratigraphic grounds.
A worked timing for a routine hit-and-run paint flake at a well-resourced Indian SFSL: microscopy and cross-section preparation in day 1; FTIR in day 2; SEM-EDX in day 3 or 4; Py-GC-MS only if FTIR is ambiguous, in week 2. Report drafted week 2 or 3. PDQ query, when triggered, adds another 3 to 6 weeks. State SFSLs without Py-GC-MS in-house refer that step to CFSL Chandigarh or Hyderabad.
The PDQ (Paint Data Query) database deserves a separate note. Maintained by the RCMP in Ottawa, PDQ catalogues automotive paint systems from OEMs worldwide by layer composition, layer count, binder chemistry and pigment elemental data. Indian SFSLs query PDQ through bilateral arrangements on flagship cases (terror-related vehicular cases, cross-border smuggling, high-profile hit-and-run prosecutions). A domestic Indian equivalent has been discussed at NFSU and the Bureau of Police Research and Development for several years; as of mid-2026 the public position is that a national paint database is in scoping phase, not yet operational. Until it is, Indian SFSLs match against in-house reference collections (typically a few hundred samples per major lab, biased to vehicles common in regional casework) and against PDQ on a case-by-case basis.